1. Field of the Invention
The present invention relates to an image heating apparatus for heating an image formed on a recording material. Examples of the image heating apparatus include, for example, a fixing apparatus for fixing an unfixed image on the recording material, and a gloss improving apparatus for improving gloss of an image by heating an image fixed on the recording material. The image heating apparatus is used in an image forming apparatus such as a copying machine, a printer, a facsimile, and a composite machine having a plurality of functions thereof.
2. Description of the Related Art
Up to now, in such the image forming apparatus, as a fixing method of fixing an unfixed toner image on a recording material, a thermal fixing method in which an unfixed toner image is heated and fused to be fixed on the recording material is generally used in view of safety and excellent fixing property.
In particular, in view of excellent thermal efficiency, easiness of down-sizing, and the like, most widely used is a heat fixing apparatus for thermally-fixing an unfixed toner image by heating and pressurizing after transporting the recording material while being sandwiched on a fixing area in which a heating rotary member and a pressure rotary member are in pressure-contact with each other.
In recent years, such the fixing apparatus becomes more diversified to allow passage of recording materials of various sizes, for example, from a relatively large-size recording material such as in A3 size to the most commonly used small-size recording material such as in A4R or B5 size. Therefore, it is necessary to configure the lengths of the heating rotary member and the pressure rotary member in an axial direction to correspond to the relatively large size such as the A3 size.
For the use of the configuration as described above, however, a non-sheet passing area in an effective fixation area of the heating rotary member, through which the recording material does not pass, becomes larger when the small-size recording material such as in A4R or B5 passes through the fixing apparatus. Then, when the small-size recording materials are continuously passed, a so-called non-sheet passing portion temperature rise phenomenon that a surface temperature of the non-sheet passing area becomes extremely high occurs because the recording material does not draw heat from the surface of the heating rotary member corresponding to the non-sheet passing area.
As techniques of coping with the non-sheet passing portion temperature rise, means 1) to 3) as described below are known.
1) A heat supply to the heating rotary member is stopped between a recording material and a recording material to idle the heating rotary member to lower the surface temperature of the heating rotary member in the non-sheet passing area to be equal to the surface temperature in a sheet passing area (throughput down control).
According to the countermeasure 1), however, since the idling for cooling the heating rotary member is required between a recording material and a recording material when the small-size recording materials are continuously passed or the like, there arises a problem in that the productivity for passing the small-size recording materials is lowered.
2) A light distribution ratio of heating means such as a heater provided in the heating rotary member is varied to make a heat quantity supplied to the non-sheet passing area smaller than that supplied to the sheet passing area for fixing the small-size recording material.
For the use of the configuration as described in 2), however, the arrangement of a heater having a plurality of light distribution ratios is required for the compatibility with a plurality of sizes, resulting in increased size of the fixing apparatus. Therefore, the number of compatible sizes is limited.
In order to cope with the problem, apparatuses described in JP S60-136779 A and JP H05-181382 A cool a non-sheet passing area with a cooling air to prevent an excessive temperature rise in the non-sheet passing area when the small-size recording material is passed.
In the configuration of the conventional example, however, the heating rotary member and a cooling fan are in communication with each other even from the start of an image forming job. Therefore, there is a possibility that an ambient temperature of the cooling fan excessively rises to thermally degrade the cooling fan. To be specific, if the ambient temperature of the cooling fan becomes higher than a resistance temperature of a sliding bearing used for the fan against oil heat or a bonding temperature of a driver IC, there arises a problem in that the cooling fan breaks down or the lifetime of the cooling fan is shortened.
As a result, the lifetime of the cooling fan is disadvantageously shortened to increase the maintenance cost required for replacing the cooling fan or the like.